This invention relates to a dual-circuit hydraulic brake system with slip control, in particular for automotive vehicles, with several sensors sensing the rotational behaviour of the wheels and with several solenoid valves controllable by slip monitoring electronics, in which brake system wheel brakes connected to the brake circuits may be pressurized by a tandem master cylinder, on the one hand, and by an auxiliary pressure source, on the other hand, said auxiliary pressure source consisting of an electric-motor-driven pressure medium pump and of an accumulator.
A hydraulic brake system with the above characteristics is known from the publication `Bosch-Technische Berichte`, Band 7, Heft 2, Seite 87, Bild 39 (`Bosch Technical Reports`, vol. 7, no. 2, page 87, FIG. 39). The known brake system comprises a tandem master cylinder with a first working chamber of the tandem master cylinder having a joint communication to the wheel brake cylinders of the rear axle of an automotive vehicle via a solenoid valve which may be switched into three positions. Via an electromagnetically operable valve, a second working chamber of the tandem master cylinder, on the one hand, communicates with a first wheel brake cylinder of the front axle of the automotive vehicle. On the other hand, it communicates with the second wheel brake cylinder of the front axle of the automotive vehicle via a further electromagnetically operable three-position valve. In the rest position of the solenoid valves, the working chambers of the master cylinder and the wheel brake cylinders are communicating freely, thus the hydraulic pressure in the wheel brake cylinders essentially corresponding to the pressures in the working chambers of the tandem master cylinder.
Slip monitoring electronics detecting the presence of a critical slip value at a vehicle wheel, the solenoid valve associated with the respective vehicle wheel will be brought into a mid-position in which communication will be interrupted between the working chamber of the tandem master cylinder and the respective wheel brake cylinder. Now, the hydraulic pressure in the wheel brake cylinder of the wheel brake that caused the critical slip value will be kept constant. Should this action of keeping the pressure constant not suffice to effect a certain reacceleration value at the respective wheel, then, the respective solenoid valve would switch into a third position in which pressure medium will be tapped from the wheel brake so as to have the braking pressure drop and to promote a reacceleration of the wheel more strongly.
The known brake system further has an electric-motor-driven pump unit disposing over two separate circuits and put into operation by an electric motor when a critical slip value will have been reached at one vehicle wheel at the least. A first circuit of the pump unit is connected to one of the working chambers of the tandem master cylinder while the other circuit of the pump unit supplies pressure medium to the other working chamber of the tandem master cylinder. Should thus a corresponding pressure reduction have averted an impending lock-up condition at one vehicle wheel, the solenoid valve associated with said vehicle wheel would switch into the rest position after having overridden the mid-position, the volume required for a new increase of the braking pressure being furnished.
The pump unit receives pressure medium from several accumulators which admittedly have a limited volume. Should there occur a rupture in one of the pressure lines of the brake system there would be a depletion of the pressure medium reserve of the accumulators within a relatively short time, thus also the pump unit failing after a correspondingly short time due to the lack of pressure fluid at the suction side.
It is thus an object of the present invention to design a dual-circuit hydraulic brake system with the above referred-to characteristics such as to prevent the pressure medium supply to a damaged brake circuit upon failure of said brake circuit while, on the other hand, ensuring the pressure medium supply to the undamaged brake circuit by the pump unit.